This invention relates to production of alumina gels and more particularly to a pseudoboehmite gel having a low sodium content.
The production of a low soda (Na.sub.2 O) pseudoboehmite gel, i.e. having a low sodium content, would be desirable from the standpoint that lower Na.sub.2 O content results in a product with a higher Brunauer-Emmett-Teller surface area and (H.sub.2 O) adsorption capacity. Less Na.sub.2 O in the gel also means a savings in production cost from less Na.sub.2 O being lost in the product and less acid, such as for example, acetic acid, being needed for pH adjustment (Na.sub.2 O removal). Furthermore, pseudoboehmite gels having high Na.sub.2 O content are not easily formed into balls, water breakage increases and the crushing strength decreases. Furthermore, a low Na.sub.2 O gel could be useful for certain catalytic applications in addition to its use as a desiccant and adsorbent.
Pseudoboehmite gel is normally produced by reacting a sodium aluminate (NaAlO.sub.2) solution with a hydrated acid salt of aluminum such as AlCl.sub.3.6H.sub.2 O, Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O, or Al(NO.sub.3).sub.3.9H.sub.2 O or with the equivalent acids themselves. However, the use of acid processes, while producing low soda gels, results in an acid salt by-product which is hard to dispose of according to environmental regulations.
Edward, Frary and Jeffries in Aluminum and Its Production, McGraw-Hill Book Company, Inc., New York (1930) at page 161 indicate that alumina can be prepared from an aluminate solution using carbon dioxide gas or sodium bicarbonate (NaHCO.sub.3). When sodium bicarbonate is used (i.e. an alkaline process), it is conventional to use a concentrated solution having approximately 80-100 grams/liter NaHCO.sub.3 to provide sufficient neutralization of the basic sodium aluminate solution. The resultant product, however, can contain about 3-6% Na.sub.2 O by total weight of the Al.sub.2 O.sub.3 (the calcined product of the pseudoboehmite gel). Quite surprisingly, however, we have now discovered that a low Na.sub.2 O pseudoboehmite gel can be produced having enhanced surface area as well, while still using an alkaline process.